A. MATHEMATICS AND ASTKONOMY. 13 



According to this theory, the aurora is the result of a second- 

 ary or induced current of electricity, produced by sudden 

 chano:es in the earth's masr-netic condition. The cause of the 

 latter changes is not yet understood ; that they exist, how- 

 ever, is abundantly proved by the automatic records of the 

 observatory at Greenwich. Glaisher's theory assumes these 

 changjes in the earth's condition as the cause of the atmos- 

 pheric currents, and of the magnetic disturbances. 



The theory of De La Rive, which for years has been con- 

 sidered the most plausible, on the contrary, considers the at- 

 mospheric currents as the primary cause, and the terrestrial 

 phenomena as the result. 3fittheil. naturforsch. Gesellschaft^ 

 JJeime, 1811, 145. 



GROUND CURRENTS AND THE AURORA OF 1872. 



The journal of the London Society of Telegraph Engineers 

 contains a very interesting summary of the phenomena of 

 " srround currents," as observed in connection with the an- 

 rora of 1872, February 4. Not only was this aurora one of 

 the most remarkable of recent years, because of the wide ex- 

 tent of its visibility, but it seems to have been attended with 

 electrical disturbances that are, as yet, unparalleled in the 

 annals of science for their intensity, if not for their duration 

 and geographical extent. The above-mentioned journal gives 

 a fac-simile of the automatic photographic records kept at the 

 Royal Observatory, Greenwich, during the day of the aurora, 

 and from this the exact nature of the electrical disturbance 

 may be deduced. It appears that all the telegraph lines of 

 the world were more or less affected on the day in question, 

 and. that all those running east and west, or nearly so, and 

 especially the ocean cables, were rendered temporarily use- 

 less for business purposes from the disturbance caused by 

 these extraneous currents of electricity, which are supposed 

 to enter the line via the earth plate, or ground connection at 

 each end of the line, and. which either overpower or partially 

 mask the effects produced by the current let on from a gal- 

 vanic battery by the operator in the ordinary course of send- 

 ing dispatches. The origin of the currents thus flowing 

 through the telegraph wires or cables from one continent to 

 the other is, so far, quite unknown, and though several hy- 

 potheses seem plausible, yet there are still wanting those 



